1,6-dibromo-2-hydroxynaphthalene is an intermediate used in the preparation of certain pharmaceutically-active compounds. For example, according to EP-A-179,447, 6-bromo-2-naphthalenes may be prepared by stoichiometric metallic reduction of the corresponding 1,6-dibromo-2-naphthalenes by the following reaction: ##STR2## in which X is hydrogen or an alkyl radical, and M is a reducing metal such as iron or tin.
The 6-bromonaphthalenes above are known and valuable compounds. For example, 6-bromo-2-methoxynaphthalene is widely used for synthesizing naproxen or nabumetone, two pharmaceuticals which are well known for their therapeutic anti-inflammatory properties and also for synthesizing methallenestril, which is an estrogen [compare The Merck Index, eleventh edition, pages 1002, 1014 and 937 (1989)].
The above dibrominated compounds may themselves be simply prepared by direct bromination of the corresponding nonbrominated compounds: ##STR3## However, reducing dibrominated derivatives of naphthalene to monobrominated derivatives by a method as described above presents the disadvantage, inter alia, of requiring large amounts of metal, which metal is ultimately present in the reaction effluent which is difficult to salvage and often becomes a pollutant, such as FeBr.sub.2.
Moreover, the yield of the desired monobrominated compounds from such a process may prove to be inadequate.
U.S. Pat. No. 5,243,088 teaches a method for treating the 1,6-dibromo compounds with nascent hydrogen and a catalyst to yield the 1-mono-dibrominated product. Also see U.S. Pat. No. 5,256,829 and U.S. Pat. No. 4,628,123. These processes disadvantageously produce copious amounts of high molecular weight by-products. Further, the selective determination is difficult to control, with by-product 2-naphthol formation diminishing yields of the desired bromo-compound.
There remains a need for carrying out the preparation of 1,6-dibromo-2-hydroxynaphthylinic compounds with high regriselectivity and minimum formation of high molecular weight by-products.